BACKGROUND: Older patients with type 2 diabetes mellitus (T2DM) have an increased risk of bone fracture independent of their bone mineral density (BMD), which is explained mainly by the deteriorated bone quality in T2DM compared to that in non-diabetic adults. Sodium-glucose co-transporter (SGLT) 2 inhibitors have been studied in several trials in T2DM, and the Canagliflozin Cardiovascular Assessment Study showed an increased fracture risk related to treatment with the SGLT2 inhibitor canagliflozin, although no evidence of increased fracture risk with treatment with other SGLT2 inhibitors has been reported. The mechanism of the difference in the fracture risk between the SGLT2 inhibitors is unknown, but the differences among the SGLT2 inhibitors in the selectivity of SGLT2 against SGLT1 may affect bone metabolism, since among the SGLT2 inhibitors the selectivity of canagliflozin is lowest. We will investigate whether the SGLT2 inhibitor luseogliflozin, which has the higher SGLT2 selectivity, affects bone metabolism by using high-resolution, peripheral quantitative computed tomography (HR-pQCT) which provides direct in vivo morphometric information about the bone microarchitecture. METHODS/DESIGN: This is a single-center, randomized, open-label, active-controlled, parallel pilot trial. Eligible participants are older (age ≥ 60 years) individuals with T2DM with HbA1c levels at 7.0-8.9%. A total of 24 participants will be allocated to either the luseogliflozin group (taking luseogliflozin) or the control group (taking metformin) in a 1:1 ratio to compare the groups' changes in bone microarchitecture of the radius and tibia which are analyzed by HR-pQCT before and at 48 weeks after the administration of each medication. The laboratory data associated with glycemic control and bone metabolism will be collected every 12 weeks during the study. Recruitment began in June 2019. DISCUSSION: The reason that we use metformin as an active control is to avoid yielding differences in glycemic control between the luseogliflozin and control groups. Besides, metformin is considered to have a neutral effect on bone. This trial should reveal the effect of luseogliflozin on bone metabolism in older patients with T2DM. TRIAL REGISTRATION:The study was registered with the University Hospital Medical Information Network (UMIN000036202) on 1 April 2019 and with the Japan Registry of Clinicla Trials (jRCTs071180061) on 14 March 2019.